High-temperature corrosion-resistant steel

ABSTRACT

THIS PATENT RELATES TO CORROSION-RESISTANT AUSTENITIC STEELSD POSSESSING IMPROVED MECHANICAL AND CHEMICAL RESISTANCE PROPERTIES AT HIGH TEMPERATURES. THE INVENTION IS MORE SPECIALLY THOUGH NOT EXCLUSIVELY DIRECTED TO SUCH STEELS HAVING IMPROVED CORROSION-RESISTANT CHARACTERISTICS IN THE PRESENCE OF VANADIUM PENTOXIDE AND WHICH WILL CONSEQUENTLY BE HIGHLY SUITABLE FOR USE IN THE CONSTRUCTION OF CERTAIN PART OF INTERNAL COMBUSTION ENGINES, INCLUDING ESPECIALLY THE VALVES THEREOF, WHICH ARE LIABLE TO BE EXPOSED TO THE COMBUSTION PRODUCTS OF LOW GRADE RESIDUAL FUEL OILS FOR THE OPERATION OF PRESENT-DAY MARINE ENGINES. THIS INVENTION DISCLOSED THAT THE ABOVE MENTIONED DESIRABLE RESULTS ARE OBTAINED IN CERTAIN CHROMIUM-CONTAINING AUSTENITIC STEEL BY USING COMPOSITIO THAT EXHIBITS THE FOLLOWING FEATURES: (1) THE USE FOR A CARBON CONTENT WITHIN A RESTRICTED RANGE OF ABOUT 0.30 TO 0.70%, (2) THE USE OF SILICON CONTENTS OF 1.10 TO 2.5%, (3) MANGANESE OF 5.0 TO 12.0%, (4) NICKEL OF 2.0 TO 7.0%, (5) CHROMIUM OF 18.0 TO 25.0%, (6) NITROGEN OF 0.20 TO 0.60%, (7) ARSENIC OF 0.02 OT 0.40%, (8) THE USE OF BORON IN AMOUNTS UP TO 0.1% , (9) THE USE OF SULFUR IN SUBSTANTIAL AMOUNTS UP TO 0.1% ALL BY WEIGHT, THE BALANCE BEING SUBSTANTIALLY IRON WITH INCIDENTAL IMPURITIES.

United States Patent i 3,592,635 HIGH-TEMPERATURE ggiIfROSION-RESISTANT ST Sueyoshi, Hatano, and Hiroshi Fujishiro,CMFl Vb Kunio Kusaka, Yokohama, Takasi Igari, Kawasaki, and Kunio Sueyoshi and Hiroshi Fujishiro, Hatano, Japan; said Sueyoshi and Fujishiro assignors t0 Nittan Valve Company, Ltd., Tokyo, Japan, and said Kusaka and Igari assignors to Tokushu Seiko Company, Ltd., Kawasaki, Japan No Drawing. Filed Mar. 4, 1969, Ser. No. 804,271 Claims priority, application Japan, Sept. 11, 1968, 43/ 64,919 Int. Cl. C22c 39/20 US. Cl. 75-128N 7 Claims ABSTRACT OF THE DISCLOSURE This patent relates to corrosion-resistant austenitic steels possessing improved mechanical and chemical resistance properties at high temperatures. The invention is more specially though not exclusively directed to such steels having improved corrosion-resistant characteristics in the presence of vanadium pentoxide and which will consequently be highly suitable for use in the construction of certain part of internal combustion engines, including especially the valves thereof, which are liable to be exposed to the combustion products of 'low grade residual fuels oils for the operation of present-day marine engines.

This invention disclosed that the above mentioned desirable results are obtained in certain chromium-containing austenitic steels by using composition that exhibit the following features: (1) The use of a carbon content within a restricted range of about 0.30 to 0.70%, (2) the use of silicon contents of 1.10 to 2.5%, (3) manganese of 5.0 to 12.0%, (4) nickel of 2.0 to 7.0%, (5) chromium of 18.0 to 25.0%, (6) nitrogen of 0.20 to 0.60%, (7) arsenic of 0.02 to 0.40%, (8) the use' of boron in amounts up to 0.1%, (9) the use of sulfur in substantial amounts up to 0.1% all by Weight, the balance being substantially iron with incidental impurities.

This invention relates to corrosion-resistant austenitic steels possessing improved mechanical and chemical resistance properties at high temperatures. The invention is more specially though not exclusively directed to such steels having improved corrosion-resistant characteristics in the presence of vanadium pentoxide and which will consequently be highly suitable for use in the construction of certain part of internal combustion engines, including especially the exhaust valves thereof, which are liable to be exposed to the combustion products of low grade residual fuel oils for the operation of present-day marine diesel engines.

This invention discloses that the above mentioned desirable results are obtained in certain chromium-containing austenitic steels by using composition that exhibit the following features: (1) the use of a carbon content within a restricted range of about 0.30% to 0.70%, (2) the use of silicon contents of 1.10 to 2.5%, (3) manganese of 5.0 to 12.0%, (4) nickel of 2.0 to 7.0%, (5) chromium of 18.0 to 25.0%, (6) nitrogen of 0.20 to 0.60%, (7) arsenic of 0.02 to 0.40, (8 the use of boron in amounts up to 0.1%, (9) the use of sulfur in substantial amounts up to 0.1%, all by weight, the balance being substantially iron with incidental impurities.

In recent years there have been developed grades of austenitic stainless steel having good corrosion-resistant properties at high temperatures, and characterized in that they include, in addition to iron, carbon, a major 3,592,035 Patented July 13, 1971 proportion of chromium, some manganese and nickel, a small proportion of nitrogen as well as silicon.

One of the commercially available valve compositions in present use today is an austenitic alloy, referred to the steel as 21-4N, which possesses a nominal commercial analysis of about 0.6% carbon, about 0.25% silicon, about 9% manganese, about 4% nickel, about 21% chromium, about 0.4% nitrogen and the balance iron. This steel has good corrosion-resistant properties in the presence of lead oxide at high temperatures.

Marine diesel engines may be made to burn a variety of fuels, for example, bunker fuel, marine fuel and residual fuel oil, thus providing a cheap source of power. It has been found that under the severe operating condition of marine diesel engines 21-4N steel are inclined to corrode much too rapidly when the combustion prodnets of vanadium oxide by the use of residual fuel oils containing vanadium are encountered at high temperatures.

The experimental Work underlying this invention has brought to light the unexpected fact that the addition of much arsenic into the steel, brings about a marked decrease in the corrosion rate of the steel at high temperatures, in vanadium pentoxide attack.

An improved steel composition according to the invention may comprise the following formulation by weight, in addition to iron and the usual impurities in the usually accepted ranges:

Percent Carbon 0.30O.70 Silicon 1.10-2.50 Manganese 5.0-12.0 Nickel 2.0-7:0 Chromium 18.0-25.0 Nitrogen 0.20-0.60 Arsenic 0.020.40 Boron Up to 0.1 Sulfur Up to 0.]

Carbon is effective for forming carbides with chromium. It has been found that at least 0.3% carbon is necessary in order to obtain the stable austenite, requisite strength and hardness. Carbon contents in excess of about 0.70% adversely affect the machinability and contribute to reducing the hot workability and ductility of the alloy. Optimum results appear to be obtained when the carbon content is maintained with the range between 0.30% and 0.70%.

Silicon improves the high temperature oxidation resistance in air also improve the resistance to high temperature corrosion caused by the presence of vanadium compounds, but silicon contents in excess of 2.50% appear to adversely affect the resistance to lead oxide of the alloy. Optimum results are obtained when the silicon content is maintained within the range between 1.10% and 2.50%.

Manganese usually contents between 0.5 and 2% for deoxidizer. Manganese materially contributes to the resistance to lead oxide and at least 5% is necessary.

Manganese contents in excess of about 12%, while being 3 when the nickel content is maintained within the range between 2.0% and 7.0%, because the nickel will possess twice as large an austenitic stability of the manganese.

Nitrogen is highly critical and is present within the alloy as an interstitial hardener. Thus it has a great influence on the strain hardening rate of the alloy. At least 0.2% nitrogen is necessary within the alloy in order to insure the high temperature strength and austenite stability. Increasing the nitrogen content to above about 0.60% results adversely affecting the hot workability, and also decreases the resistance to vanadium pentoxide of this alloy. Optimum results appear to be obtained when the nitrogen content is maintained within the range of about 0.20% and 0.60%.

Chromium is present within the range between 18% and 25%. Within this range the alloy possesses the required degree of corrosion resistance, especially in atmospheres containing combustion products of leaded and containing vanadium fuels. In addition, at least 18% chromium is necessary in order to alford the alloy an acceptable measure of resistance to oxidation at elevated temperatures, but the use of an amount too great must be avoided in order to prevent the decrease of hot workability. The most satisfactory combination of properties occurs when the chromium content is within the range between 18.0% and 25.0%.

It has been discovered, by this inventor, that addition of arsenic brings about a marked decrease in the corrosion rate of the atmospheres containing vanadium pentoxide. At least 0.02% arsenic is necessary within the alloy in order to insure this properties. Amount of arsenic much beyond 0.40% tend to worsen hot workability and resistance of the steel to corrosion by lead oxide, so that it is preferable that the arsenic content is maintained within the range of about 0.02% and 0.40%.

As respects boron, the use of this element is optional, but to provide better creep strength, it is desirable to include an effective addition of boron. Amounts of boron up to 0.1% have a favorable effect upon creep strength, but boron additions of more than about 0.04% tend to worsen resistance of the steel to high temperature corrosion, so that it is preferable that the boron not exceeding 0.025%.

As respects sulfur, a noticeable improvement in machinability is obtained with amounts of sulfur up to 0.1%.

Moreover, the steel can contain other elements beyond the range indicated above, such as vanadium not exceeding 2%, tungsten not exceeding 2%, molybdenum not exceeding 2%, columbium not exceeding 1%, titanium not exceeding 1%, cobalt not exceeding 5% and Zirconium not exceeding 0.3%.

The following Table 1 presents the composition tested in this invention.

TABLE l.CHl lMICAL COMPOSITION OF STEELS TESTED Chemical composition (percent) C Si Mn S Ni Cr N AS B The steel, designated A in this table is equivalent to 21- 4N steel, and B is 214N steel containing 0.21% arsenic, while C, D, E, F and G are the steels of the present invention. These steels were aged for 6 hours at 750 C. after solution treated at 1150 C.

Table 2 shows the results of vanadium attack tests by melted vanadium pentoxide containing 10% sodium 4 sulfate at 900 C. The steels of our invention are considerably superior to steel A and steel B in its resistance corrosion caused by the presence of vanadium compounds.

TABLE 2.-RESULTS OF VANADIUM ATTACK TEST AT 900 C.

Steel No.: Weight loss (gr./dm. /hr.

A 25.0 B 14.0 C 10.2 D 8.2 E 8.0 F 7.5 G 6.9

Table 3 shows the results of oxidation test at 900 C. for 200 hours in air.

TABLE 3.-RESULTS OF OXIDATION TEST AT 900 C. FOR 200 HOURS IN AIR Steel No.: Weight increase (gr/m?) A 30.5 B 29.5 C 22.5

The steels of our invention are considerably superior to 21-4N steel in its high temperature oxidation resistance in air.

Table 4 shows the hardness aged for 6 hours at 750 C. after solution treatment at 1150 C.

The steels of our invention are slightly harder than 21-4N steel.

The results of tensile test at room and elevated temperature are given in Table 5. Specimens are solution treated at 1150 C. for 1 hour, oil quenched, and aged at 750 C. for 6 hours.

TABLE 5.TENSILE TEST RESULTS Tensile Percent strength Temperature (kg/mmfl) Elongation Reduction Steel No.:

{Room 114 19.0 20.0 800 C. 38.5 28.5 46.5 F fRoom 116 18.0 19. 0 l800 C 38. 8 26. 0 44. 5

Table 6 also shows the Charpy Impact Test at 800 C. for each of the steels. The steel of the present invention has the strength and ductility that shows in each case with the 21-4N steel.

TABLE 6.IMPACT STRENGTH AT 800 C.

Steel No.: Charpy impact value (kgm./cm. A 4.9

Table 7 hows the results of corrosion test by lead oxide at 916 C.

TABLE 7.-RESULTS OF CORROSION TEST BY LEAD OXIDE AT 916 C.

Silicon improves the corrosion resistance to vanadium attack, but impairs the lead oxide corrosion resistance. Higher percentages of silicon beyond 1.84% are accompanied by a marked increase of weight loss in the presence of molten lead oxide.

Silicon content in the steel of our invention in amount less than 1.%, considerably improves the resistance to corrosion caused by the presence of vanadium compounds without impairing the corrosion resistance to molten lead oxide.

As many described, this invented steel that is added much arsenic than incidental impurities to 2l-4N steel having a relatively high silicon content, is characterized by exhibiting excellent resistance to vanadium pentoxide attack at elevated temperatures. This steel is suitable for use of diesel engine valve, valve parts and other internal combustion engine components because it has also a strength at room temperature and high temperatures. The steel of our invention containing 1.10-1.50% silicon is suitable for the gasoline engine exhaust valve which are liable to be exposed to the combustion products of lead tetraethyl as present in high-grade present-day fuels.

What is claimed is:

1. An austenitic steel composition consisting essentially of substantially 0.30-0.70% carbon, 1.10-2.50% silicon, 5.0l2.0% manganese, 2.07.0% nickel, l8.025.0% chromium, 0.20 O.60% nitrogen, 0.020.40% arsenic, up to 0.1% boron and up to 0.1% sulfur, all by weight, the balance being substantially iron, and characterized by its corrosion resistance properties in the presence of vanadium pentoxide, at high temperatures.

2. An austenitic steel composition consisting essentially of substantially 0.30-0.70% carbon, 1.1-2.5% silicon, 5.0- 12.0% manganese, 2.0-7.0% nickel, 18.025.0% chromium, 0.'200.60% nitrogen and 0.020.40% arsenic, all by weight, the balance being substantially iron, and characterized by its high corrosion resistance properties in the presence of vanadium pentoxide, at high temperatures.

3. An austenitic steel composition consisting essentially of substantially 0.300.70% carbon, l.10l.50% silicon, 5.0l2.0% manganese, 2.0-7.0% nickel, 18.025.0% chromium, 0.200.60% nitrogen, 0.020.40% arsenic, up to 0.1% boron and up to 0.1% sulfur, all by weight, the balance being substantially iron, and characterized by its high corrosion resistance properties in the presence of lead oxide and vanadium pentoxide, at high temperatures.

4. A corrosion-resistant composition of austentic steel consisting essentially of, by weight, substantially 0.30- 0.70% carbon, 1.102.50% silicon, 5.0l2.0% manganese, 2.0-7.0% nickel, 18.0-25.0% chromium, 0.20 0.60% nitrogen and 0.020.40% arsenic, the balance being substantially iron.

5. High temperature corrosion-resistant austenitic steel composition consisting essentially of substantially 0.40- 0.60% carbon, LTD-2.50% silicon, 8-l0% manganese, 4-5 nickel, 20-22% chromium, 0.3-0.5 nitrogen and 0.1-0.3% arsenic by weight, the balance being substantially iron.

6. Cast article for use as a part of an intrenal combustion engine and composed of a high-temperature corrosionresistant austenitic steel consisting essentially of substantially 0.30-0.70% carbon, 1.l0-2.50% silicon, 5.0l2.0% manganese, 2.0-7.0% nickel, 18.025.0% chromium, 0.20-0.60% nitrogen and 0.02-0.40% arsenic, all by weight, the balance being substantially iron.

7. Exhaust valve for use in an internal combustion engine, composed of a high-temperature corrosion-resistant austenitic steel consisting essentially of substantially 0.300.70% carbon, 1.102.50% silicon, 5.0l2.0% manganese, 2.0-7.0% nickel, 18.025.0% chromium, 0.20- 0.60% nitrogen and 0.02-0.40% arsenic, all by weight, the balance being substantially iron.

References Cited UNITED STATES PATENTS 2,053,846 9/1936 Schulz 123A 2,602,738 7/1952 Jennings 75-128A 3,152,934 10/1964 Lula 75--128N 3,301,668 1/1967 Cope 75128R HYLAND BIZOT, Primary Examiner U.S. Cl. X.=R. 

